Fluorescent lamps have become widely used in the past several years due to their relatively low power consumption and high light output, especially as compared with incandescent type lamps. With the general expansion in the use of fluorescent lamps, fluorescent lamps have found their way into a wide variety of applications, including high and low temperature environments. Fluorescent lamps have also become the dominant lighting fixture for offices, factories and other businesses.
Solid state ballasts which drive fluorescent lamps are similar to the prior art shown in FIG. 1 of the attached drawings, which is similar to the ballast shown in U.S. Pat. No. 4,935,862. Ballasts frequently consist of a rectifier to convert an incoming alternating current (AC) signal to a high voltage direct current (DC) signal which is then used to drive the fluorescent lamps. A full wave rectifier, as shown in FIG. 1, is fairly common, as is a half-wave rectifier (not shown). A pulsed DC voltage that is output by this AC to DC converter is applied across a fluorescent lamp in order to excite the gas contained within the fluorescent lamp, which causes the lamp to light. A feedback circuit is generally associated with the fluorescent lamp, causing the DC signal to pulse, and the voltage across the fluorescent lamp to oscillate. A starter circuit is usually provided to supply a start pulse to begin the oscillation of the DC voltage.
The voltage applied across a fluorescent lamp must be sufficient to excite the atoms and generate light. Typically, a voltage level of approximately 110-130 volts is sufficient to cause a five foot long T8 fluorescent lamp to light. When operating in a low temperature environment, however, the voltage which must be applied across the fluorescent must be higher because the low temperature of the gas, and resulting decreased entropy of the atoms requires a higher energy in order for the atoms to become excited enough to generate light. The impedance of a fluorescent lamp at a given temperature remains fixed and the higher energy can only be supplied in the form of a higher voltage across the electrodes in the fluorescent lamps.
Another problem which compounds the low temperature ignition difficulties of a fluorescent lamp is parasitic resistances which may develop between the fluorescent lamp and ballast driving the fluorescent lamp. These parasitic resistances can take the form of deterioration of the wires going to the ballast causing an increased resistance and the like. In addition, certain types of fluorescent lamp ballasts use components which are temperature sensitive. Significant changes in the operating temperature of the ballast requires the circuit to be redesigned, or retuned to accommodate the changed circumstances.
Another disadvantage of fluorescent tubes is a stroboscopic effect or flickering. Using classic ballasts in which the incoming 50 or 60 Hz AC signal is routed directly to the fluorescent lamp, the luminous arch is ignited and turned off with a frequency of double the supply frequency, i.e. 100 or 120 Hertz. This stroboscopic effect is usually not visible, but may under adverse circumstances especially at cooler temperatures cause inconvenience. Furthermore, acoustic noise is often induced, particularly by the induction coil, and the usual simple ignition device may cause slow ignition involving several attempts to ignite the lamp accompanied by an unpleasant flicker.
These problems frequently go unnoticed because the user of a fluorescent lamp and ballast will frequently change the fluorescent lamp when a problem develops. Changing the fluorescent lamp involves releasing electrical connectors which are attached to the fluorescent lamp and reconnecting the connectors to a new fluorescent lamp that has no corrosion on the connectors and has not yet suffered any degradation of the electrode elements within the lamp itself. This results in replacing fluorescent lamps much more frequently than is necessary, and in discarding fluorescent lamps which would be usable in a room temperature application or would be usable with a better ballast. This results in the production of a large amount of waste.
If replacing the fluorescent lamp with a new lamp does not solve the particular problem, the entire ballast is replaced, usually at a much greater cost than replacing the lamp. This may solve the problem temporarily or may result in successive replacement of the ballast and lamp until an optimum match is achieved between a particular ballast and lamp which produces results acceptable to the user.